1. Field of the Invention
The present invention generally relates to a CVD apparatus for forming a thin film having a high dielectric constant, and more specifically to a CVD apparatus for forming a variety of thin films, in particular a thin dielectric film such as is used in a semiconductor memory, by Chemical Vapor Deposition (CVD) method. The present invention also relates to a method of forming such dielectric thin film using the CVD method.
2. Description of the Background Art
In recent years, there has been a rapid advancement in the integration of semiconductor memory and devices. For instance, device capacity (number of bits) in a dynamic random access memory (DRAM) has quadrupled in three years. This integration aims at achieving the reduction in size of a device, lower power consumption, and lower cost, and so on. Regardless of the improvement in integration, however, a capacitor, being a DRAM component, must be able to accumulate a certain amount of electric charges. Thus, along with the increase in integration of a device, attempts have been made to minimize the thickness of a capacitor dielectric film or to increase the area of a capacitor by making its shape complex.
Nevertheless, it has become difficult to reduce the film thickness of a conventional capacitor with SiO.sub.2 as its main dielectric material. Instead, as a noted alternative measure for increasing storage charge density, the dielectric film material of a capacitor may be replaced with film material having a higher dielectric constant. By using high dielectric constant material, an increase in storage charge density is achieved which is comparable to that obtained by the conventional method of reducing film thickness. Moreover, if a thin film with a high dielectric constant can be used, the film can be of a certain thickness, and the use of a high dielectric constant material may provide advantages with regard to film deposition processes and film reliability.
Most importantly, it is required that such a capacitor dielectric film be a thin film with a high dielectric constant as described above and have small leakage current. The desirable target values for these characteristics, in general, are considered to be approximately 0.5 nm or below for film thickness in SiO.sub.2 equivalent and 2.times.10.sup.-7 A/cm.sup.2 or below for leakage current density at the voltage application of 1V.
As such, oxide type dielectric films including tantalum oxide, lead zirconate titanate (PZT), lead lanthanum zirconate titanate (PLZT), strontium titanate (ST), barium titanate (BT), and barium strontium titanate (Ba, Sr) TiO.sub.3 (hereafter referred to as BST) seem promising. Moreover, several methods have been devised for producing these thin films, and are being put to practical use experimentally.
Generally, to form a thin film on an electrode for a capacitor of a DRAM having minute steps, film deposition employing a CVD method which provides good coverage to surfaces having complex shape is most advantageous in simplifying the process. In a CVD method, organometallic compound containing a given metal is used as the thin film source having a high dielectric constant. By vaporizing the source and spraying the resulting gas onto a substrate, a thin film with a high dielectric constant is formed. It has been a big problem, however, that a CVD source with a stable and good vaporization characteristic does not exist. This is largely due to the unsatisfactory vaporization characteristic, by heating, of the compound of metal and .beta.-diketon-type dipivaloylmethane (hereinafter referred to as DPM) frequently used as a CVD source.
It was under such circumstances that the applicants proposed in the Japanese Patent Laying-Open No. 7-268634 a CVD source being produced by dissolving a conventional solid material in organic solvent called tetrahydrofuran (THF) and thus having a greatly enhanced vaporization characteristic. Further, a CVD apparatus using liquid source was developed which vaporizes the liquid source and supplies it stably to a reaction chamber. They have also found that this apparatus can be utilized for depositing a high dielectric constant thin film having good surface morphology and electrical characteristics.
Even the use of this CVD apparatus using liquid source, however, has been discovered as not being capable of providing a dielectric film with good stable characteristics that last a long time. Upon examination, it has become apparent that this problem is caused by the very small amount of vaporization residue produced in the vaporization process.
Also, it has been discovered that the vaporizer for making a liquid source proposed by ATM Co. Ltd. in the United States (U.S. Pat. No. 5,204,314) does not provide sufficiently stable film deposition due to the formation of a solid in the portions where the source vaporizes and the consequent blocking of the tubes.
The arrangement of the conventional CVD apparatus using liquid source will be described below.
FIG. 5 is a schematic diagram depicting the representation of a conventional CVD apparatus using liquid source. Here, an example is given in which the BST film is deposited using reactive gas O.sub.2 and the liquid source having solid Ba (DPM).sub.2, Sr (DPM).sub.2, and TiO (DPM).sub.2 dissolved in THF. The CVD apparatus includes a source gas supply tube 1, a reactive gas supply tube 2, and a reactor 3. A heating stage 4 is provided in the reactor 3. A susceptor 5 is provided on the heating stage 4. The susceptor 5 supports a substrate 6. A diffusion board 7 is provided in the upper portion of the reactor 3. Pressure gauges 8a, 8b are provided in the reactor 3. An exhaust passage 11 is connected to the reactor 3. A vacuum valve 9 and a pressure controller 10 are provided somewhere along the exhaust passage 11. The CVD apparatus also includes a vaporizer 21, a vaporizer heater 22, a constant temperature box 23, a tube heater 24, and a mixer 25.
The N.sub.2 gas 13a having its amount controlled by a gas flow rate controller 16 flows through a connection tube 26 into the vaporizer 21. Ba (DPM).sub.2 /THF in a liquid source vessel 17 is pressurized by the N.sub.2 gas 13a through a pressure tube 14, has its amount controlled by a liquid flow rate controller 15, and is sent into the vaporizer 21 through the connection tube 26. Sr (DPM).sub.2 /THF in a liquid source vessel 18 is pressurized by the N.sub.2 gas 13a through a pressure tube 14, has its amount controlled by a liquid flow rate controller 15, and is sent into the vaporizer 21 through the connection tube 26.
TiO (DPM).sub.2 /THF in a liquid source vessel 19 is pressurized by the N.sub.2 gas 13a through a pressure tube 14, has its amount controlled by a liquid flow rate controller 15, and is sent into the vaporizer 21 through the connection tube 26.
THF in a liquid source vessel 20 is pressurized by the N.sub.2 gas 13a through a pressure tube 14, has its amount controlled by a liquid flow rate controller 15, and is sent into the vaporizer 21 through the connection tube 26.
Next, the operation will be described.
The N.sub.2 gas 13 having its flow rate regulated by the gas flow rate controller 16 flows through the connection tube 26. The solution sources in liquid source vessels 17, 18, 19, 20 pressurized by the N.sub.2 gas 13a through pressure tubes 14 are provided into the connection tube 26, and having their amount controlled by the liquid flow rate controllers 15, are supplied to the vaporizer 21. Thereafter, the supplied liquid sources run into a large area of the inner wall of the vaporizer 21 heated by the vaporizer heater 22 and instantly vaporize. The vaporized sources inside the vaporizer 21 pass through the source gas supply tube 1 heated by the constant temperature box 23 and the tube heater 24 and are supplied into the reaction chamber 3a. The reactive gas 2b, on the other hand, passes through the reactive gas supply tube 2 heated by the constant temperature box 23 and the tube heater 24 and is supplied into the reaction chamber 3a. The source gas and the reactive gas are introduced into the reaction chamber 3a only after they are mixed by the mixer 25. Finally, the source gas and the reactive gas react on the substrate 6 such as a silicon substrate heated by the heating stage 4 to form a BST film. Moreover, the mixed gas which failed to contribute to the formation of a thin film is exhausted by a vacuum pump through the exhaust passage 11.
The pressure in the reaction chamber 3a is controlled to be between 1 and 10 Torr by the pressure controller 10. Since lower temperature provides better step coverage, the temperature of the heating stage 4 is set to 400 to 600.degree. C. By controlling the source flow rate and the duration of film deposition, a film is deposited at the rate of about 30 .ANG./min, with the film thickness of 300 .ANG. and the BST film composition ratio of (Ba+Sr)/Ti=1.0. An upper electrode is formed by sputtering Pt or Ru on the BST film formed on a lower electrode made of materials such as Pt, Ru, or the like. This sample is used to measure the electrical characteristics of the BST film, such as leakage current and oxide film equivalent film thickness.
The following problems have been recognized when depositing film with the conventional CVD apparatus using liquid source using the above-mentioned CVD source.
First, when introducing the CVD source gas vaporized in the vaporizer and a reactive gas such as O.sub.2 gas which is an oxidizing agent into the reaction chamber, both gases cool down as soon as they enter the reaction chamber. Thus, the organometallic compound dissolved in organic solvent precipitates before reaching the substrate. This either remains as residue in tubes and on inner walls of the reaction chamber, or scatters as fine particles, which mix into the device and cause a defect in the device.
Second, when introducing the CVD source gas vaporized in the vaporizer and a reactive gas such as O.sub.2 gas which is an oxidizing agent into the reaction chamber, the two are conventionally mixed by a mixer. One of the gases, however, possibly flows backward into a supply tube of the other gas, and the reaction occurs somewhere along the respective gas supply tubes. As a result, the organometallic compound reacts inside the gas supply tubes and the vaporizer which are upstream of the mixer, precipitate forms, and fine particles of the vaporization residue or the reaction product in gaseous phase are produced.
Third, due to the formation of residue as a result of the reaction of the organometallic compound somewhere along the respective gas supply tubes, the conductance of the supply tubes declines, while the internal pressure of the vaporizer increases. Consequently, problems arise where residue forms at an increasing speed, blocking the supply tube, thereby requiring frequent cleaning of the supply tubes.
As described above, inside the conventional CVD apparatus for vaporizing solution, a foreign substance such as the residue of a CVD source would adhere to the vaporizer, the source gas supply tube, the mixer, and so on, causing the pressure of the vaporizer to rise and thus causing more deposition of the residue. In addition, the fine particles which are generated at the same time are taken in by the film being deposited, and become a cause of defect in the device.